Part 3 : NATURE of the PLANET




Distances to Neptune from our Sun averages some 29.9 A.U. or 4.3 billion kilometres, being about two-thirds (19.2/29.9 AU) further away than the similar sized planet of Uranus. From the orbit of Neptune, our Sun appears like a very bright star of -19 magnitude, whose very small apparent diameter subtends around 1.1 arcmin. This may varies in size throughout the orbit by only about 2% to 3%. Indeed, Neptune is so far from the Sun that the average observed diameter is merely 2 arcsec or only 11800th of Earth (1°). We find that that the very small brightness differences between each opposition and conjunction are just 0.2 magnitude (7%.)


Discoverer : Johann Gottfried Galle
Discovery Date : 23rd September 1846
Predictions : John Adams & Urbain Leverrier
Satellites : 13 (2006)
Equatorial : 49,532 ± 30 km.
Polar : 48,684 ± 60 km.
Flattening : 0.0171
Polar Tilt : 29.56°
Period (P) : 164.79132 years
Synodic Period : 367.49 days
Orbital Velocity 5.48 km.s-1
Eccentricity (e) : 0.0113
Inclination (i) : 1.769 17°
Mass : 1.024 ×1026 kg.
Mean Density : 1.638
Mean Distance (a) : 4.34731 × 109 km.
Mean Distance (AU) : 29.9
Sidereal Rotation : 16.7±1.4 hr.
Mean Sidereal Rotation : 16.11±0.01 hr.
(magnetic) Day Length : 16.11 hr.
Maximum Diameter : 2.4″ (arcsec)
Minimum Diameter : 2.2″ (arcsec)
Maximum Magnitude : +7.84

Neptune in its long orbit takes some 164.79 years (164 years 10 month 17.0 days) to completely encircle the Sun, crossing on each constellation of the zodiac approximately averaging every fourteen years. Since found in 1846, it has yet only just complete one whole orbit. We recently celebrated this on Sunday 10th July 2011), where the distant planet returned to an almost identical place within the constellation of Aquarius. Exact positions were not exactly the same because of these same small perturbations that was noticed in the original discovery of Neptune have slightly changing the orbit. Its second orbit since discovery will again complete on 24th August 2176.

For many years our knowledge was thwarted by even the largest telescopes discovering any of Neptunes secrets — including properties as the diameter, composition and nature. Little was known about Neptune, until Voyager 2 had a brief encounter with the planet during August 1989. We knew only that the atmosphere was methane-rich, accounting for its observed blue-green colour. Methane happens to absorbs much of the faint red sunlight, which makes the visual rich blue colour. First impressions sometimes convince novices that its appearance looks very similar images of the watery Earth from space Earth. In fact, there is no true water or any ocean-like familiarity here. Perhaps the only thing in common with Earth is the axial tilt of 29½°, but here the resemblance stops.

Neptunes equatorial diameter is now known to be 44,000 kilometres. This makes it just a fraction smaller than Uranus, but it is still four times the Earth diameter. By mass, it is about 17 times heavier than Earth but is about one-quarter less dense. Much of the planetary atmosphere was found to be composed of about 84% Hydrogen and 15% Helium, with the Methane content being the remaining about 3%. There had always some suspicion of faint belts across the planetary disk, and as the Voyager 2 drew closer, all was soon revealed.

Neptune HST Neptune HST
Image NASA, ESA, STScI (1st September 2005).
Fig. 3. (right) NEPTUNE OBSERVED by HST
Image NASA, ESA and the Hubble Heritage
Team (STScI/AURA) (12th July 2011)

Here many new discoveries were made. Perhaps one of the most startling discovery was the aptly named Great Dark Spot (GDR) on the cloud-base, and faint cirrus like clouds of methane appearing above Neptune cloud-based surface. During 2003/04 observations by the 10-metre Hawaiian Keck II telescope had found that this large spot had vanished. It was further examined in September 2005 when the Hubble Space Telescope (HST) was reexamine the planet from afar. They then found the similar circumstances with the now totally missing GDR, but found new changes. (Fig. 2). On 12th July 2011 another set of images was taken by HST, and this time, they found complicated cloud patterns had developed in the Neptunian atmosphere.

Why Neptune has such an variable and active atmosphere so far from the Sun is still not presently fully understood. It is speculated that Neptune may have seasons that change over 164 years, depending on place in the orbit

A similar magnetic field strengths to Earth surrounds Neptune, being was found to be unusually tilted by some 50° against the rotational axis. The atmospheric daily rotational period of was also discovered to be about 16½ hours, being slightly slower than first thought. Intriguingly, the planet was found to be more active than ever imagined — surprising after Voyager 2 visit to the then apparently quite inactive atmosphere of Uranus.


In December 2004, NASA had announced that it was considering to launch a new exploratory mission to Neptune, to both study and attempt to understand the planetary conditions so far from the Sun. To be called the “Neptune Triton Orbiter, NASA was expected to launch the craft around 2014, whose nuclear-electric propulsion system would had be required to get there. So far from the Sun means that the avalable solar power does not work in the outer depths of the Solar System. To be captured and then orbit Neptune, this spacecraft would need to travel through the Solar System helped by many gravity assists from the inner planets and Jupiter, before finally reaching the right trajectory and velocity to place the spacecraft an orbit in 2035. This project and plan has since been scapped for another day.


  Conjunction      |       Opposition  
dd mon year  UT  AEST | dd mon year  UT  AEST
05 Feb 2006  23h 09h* | 10 Aug 2006  22h 08h*
08 Feb 2007  10h 20h  | 13 Aug 2007  11h 21h 
10 Feb 2008  20h 06h* | 15 Aug 2008  01h 11h 
12 Feb 2009  07h 17h  | 17 Aug 2009  14h 00h*
14 Feb 2010  17h 03h* | 20 Aug 2010  03h 13h 

17 Feb 2011  20h 06h* | 22 Aug 2011  22h 08h*
19 Feb 2012  08h 18h  | 24 Aug 2012  11h 21h 
21 Feb 2013  04h 14h  | 27 Aug 2013  00h 10h 
23 Feb 2014  15h 01h* | 29 Aug 2014  13h 23h 
26 Feb 2015  13h 23h  | 01 Sep 2015  13h 02h 

28 Feb 2016  14h 00h* | 02 Sep 2016          
02 Mar 2017           | 05 Sep 2017          
04 Mar 2018           | 08 Sep 2018          
07 Mar 2019           | 10 Sep 2019          
09 Mar 2020           | 12 Sep 2020          
No Neptune Aphelion / Perihelion Events
occurs between these dates
      * Event Occurs in AEST the Next Day         

Disclaimer : The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. Onus of its use is placed solely with the user.


Last Update : 08th August 2015

Southern Astronomical Delights © (2012)

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